Light emitting diode, light emitting device using the same, and fabrication processes therefor

ABSTRACT

Disclosed is a LED which can be mounted at high density on a large area display. Having a hole for heat sink in the ceramic substrate, the LED is superior in heat sink property. In order to fabricate the light emitting device, first, a secondary ceramic sheet is stacked on the ceramic substrate, followed by forming electrodes in a predetermined pattern on the secondary ceramic sheet around the hole for heat sink. On the ceramic substrate, an upper ceramic sheet with an opening is stacked to form a stacked ceramic substrate in such a way that a part of the electrodes are exposed through the opening. After co-firing the stacked ceramic substrate, a light emitting diode chip is mounted on the secondary ceramic sheet at a position corresponding to the hole for heat sink. Then, the electrodes are electrically connected with the LED chip, and the LED chip is sealed with insulating resin. A light emitting device using the LED and a fabrication method therefor are also disclosed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a light emitting diode(hereinafter referred to as “LED”), and a fabrication process therefor.More particularly, the present invention relates to a high-densitymounting LED suitable for use in large area light emitting devicedisplay and illuminating facilities and a fabrication process therefor.Also, the present invention relates to a light-emitting device using theLED with a superior heat sink property and a fabrication processtherefor.

[0003] 2. Description of the Prior Art

[0004] LED is one kind of solid lighting indicators. LEDs for singlecolors including RGB (red, green and blue) colors, three primary colorsof light, have been followed by LEDs for W (white) colors, which haveutilized in more various fields. Recently, LEDs have been developed fromlamp types to surface mount device (SMD) types that allow LEDs to bemounted at so high a density as to make a large area display. Accordingto such a tendency, LEDs are used in a broader applications, fromindicators to back light sources for LED displays, further to anext-generation lighting system which is substitutive for conventionallighting system such as incandescent electric lamps, fluorescent lampsor street lamps. Unlike general lamps, lighting systems using LEDs havesimple lighting circuits and need not invert circuits nor core typeballasts. Over incandescent lamps, LED luminaire has the advantage ofbeing lower in maintenance cost because of its being operated at lesselectrical power, with 10-times longer lifetime.

[0005] Representative example of white LED applicable to luminaire isdisclosed in Japanese Pat. Laid-Open Publication No. 2000-315826 whichdescribes a light emitting device composed of an LED and a phosphor. Thelight emitting device of this patent, as shown in FIG. 1a, comprises ablue LED chip 3 mounted on a ceramic substrate 1, a first transparentcoating 6 covering the blue LED chip 3, a second transparent coating 6 apositioned on the first transparent coating 6, and an electrodeelectrically connected to the LED chip 3 via a wire 5, the secondtransparent coating 6 a containing a phosphorescent material. In thisstructure, the fluorescent material absorbs the visible light beamemerging from the LED chip to radiate fluorescent light so that mixingthe visible light of the LED chip with the fluorescent light emittedfrom the fluorescent material results in emission of white light. Thislight emitting device can emit homogenous white light with excellentefficiency. Another LED which is able to emit white light may be foundin relevant literature (e.g., U.S. Pat. Nos. 5,998,925 and 6,069,440).

[0006] Generally, appliances with these LEDs are known deteriorate inlight emitting properties mainly owing to thermal stress. In a lightingsystem or a traffic signal, which is fabricated by mounting a multitudeof conventional LED chips on a substrate at a high density, as shown inFIG. 1b, LEDs suffer from more overheating than LEDs in any otherapplications and have a tendency to radiate heat in proportion to thetotal lighting area. Particularly in the case of blue LEDs, the heatingproblem is more serious because they are operated at higher voltagescompared to LEDs of other colors. Further, the larger areas of LEDluminaire aggravate the performance and out-of-order rate of the LEDsused. This trend is also found when LEDs are mounted at higher densitiesin a given area. Conventional light emitting devices, most of which arestructured as illustrated in FIG. 1b, are so poor in heat sinkcapability as to show a limit in mounting LED chips with a high densityin a large area.

SUMMARY OF THE INVENTION

[0007] Therefore, it is an object of the present invention to provide anLED which can be mounted at high density in a large area with a superiorheat sink property.

[0008] It is another object of the present invention to provide a methodfor fabricating such an LED.

[0009] It is a further object of the present invention to provide alight emitting device fabricated with the LEDs, which shows high heatsink properties even with a large area.

[0010] It is still a further object of the present invention to providea method for fabricating such a light emitting device.

[0011] It is still another object of the present invention to provide alight emitting unit assembly for large area using the light emittingdevices.

[0012] In an aspect of the present invention, there is provided a lightemitting diode, comprising: a ceramic substrate having a hole for heatsink, the ceramic substrate being formed electrodes in a predeterminedpattern thereon around the hole; a secondary ceramic sheet covering thehole for heat sink to mount a LED chip; a LED chip mounted on thesecondary ceramic sheet over the hole to be electrically connected withthe electrodes through wires; an upper ceramic sheet formed on theceramic substrate, the upper ceramic sheet surrounding the LED chip; andan insulating layer formed to seal the LED chip within the upper ceramicsheet.

[0013] In accordance with another aspect of the present invention, thereis provided a method for fabricating a light emitting diode, comprisingthe steps of: preparing a ceramic substrate having a hole for heat sink,the ceramic substrate being formed electrode patterns in a predeterminedpattern thereon around the hole; stacking a secondary ceramic sheet onthe ceramic substrate to cover the hole for heat sink; stacking on theceramic substrate an upper ceramic sheet with an opening to form astacked ceramic substrate in such a way that a part of the electrodepatterns and a part or all of the secondary ceramic sheet are exposedthrough the opening; co-firing the stacked ceramic substrate; mounting aLED chip on the secondary ceramic sheet at a position corresponding tothe hole after disposing electrodes on the electrode patterns of theceramic substrate; and sealing the LED chip within the upper ceramicsheet with an insulating resin after electrically connecting theelectrodes with the LED chip.

[0014] In accordance with a further aspect of the present invention,there is provided a light emitting device, comprising: a ceramicsubstrate having a plurality of holes for heat sink, the ceramicsubstrate being formed electrodes in a predetermined pattern thereonaround each of the holes; secondary ceramic sheets covering each of theholes for heat sink to mount LED chips; a plurality of LED chips mountedon the secondary ceramic sheets over each of the holes to beelectrically connected with the electrodes through wires; an upperceramic sheet formed on the ceramic substrate, the upper ceramic sheetsurrounding the LED chips; and an insulating layer formed to seal theLED chips within the upper ceramic sheet.

[0015] In accordance with still a further aspect of the presentinvention, there is provided a method for fabricating a light emittingdevice, comprising the steps of: preparing a ceramic substrate having aplurality of holes for heat sink, the ceramic substrate being formedelectrode patterns in a predetermined pattern thereon around each of theholes; stacking secondary ceramic sheets on the ceramic substrate tocover each of the holes for heat sink; stacking on the ceramic substratean upper ceramic sheet with an opening to form a stacked ceramicsubstrate in such a way that a part of the electrode patterns and a partor all of the secondary ceramic sheets are exposed through the opening;co-firing the stacked ceramic substrate; mounting LED chips on thesecondary ceramic sheets at a position corresponding to each of theholes after disposing electrodes on the electrode patterns of theceramic substrate; and sealing the LED chips within the upper ceramicsheet with insulating resin after electrically connecting the electrodeswith the LED chips.

[0016] In accordance with still another aspect of the present invention,there is provided a large area light emitting assembly, composed of amultitude of the light emitting devices.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0018]FIG. 1a is a schematic cross sectional view showing a conventionalLED;

[0019]FIG. 1b is a schematic cross sectional view showing a lightemitting device using the LED of FIG. 1a;

[0020]FIG. 2 schematically shows the structure of an LED in a crosssectional view and a plane view, in accordance with an embodiment of thepresent invention;

[0021]FIG. 3 schematically shows LEDs in cross sectional views, inaccordance with embodiments of the present invention;

[0022]FIG. 4 schematically shows LEDs in cross sectional views, inaccordance with other embodiments of the present invention;

[0023]FIG. 5 schematically shows the structure of a light emittingdevice in cross sectional view and a plane view, in accordance withanother embodiment of the present invention;

[0024]FIG. 6 schematically shows light emitting devices in crosssectional views, in accordance with still other embodiments of thepresent invention;

[0025]FIG. 7 schematically shows light emitting devices in crosssectional views, in accordance with yet other embodiments of the presentinvention;

[0026]FIG. 8 illustrates processes for fabricating a light emittingdevice in plane views and corresponding cross sectional views, inaccordance with another embodiment of the present invention;

[0027]FIG. 9 illustrates processes for fabricating a light emittingdevice in plane views, in accordance with another embodiment of thepresent invention;

[0028]FIG. 10 illustrates processes for fabricating a light emittingdevice in plane views and corresponding cross sectional views, inaccordance with a further embodiment of the present invention;

[0029]FIG. 11 illustrates processes for fabricating a light emittingdevice in plane views, in accordance with another embodiment of thepresent invention;

[0030]FIG. 12 shows examples of upper ceramic sheets applicable for thepresent invention;

[0031]FIG. 13 is a schematic view showing the structure of a large arealight emitting unit assembly, composed of LEDs of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Leading to the present invention, the intensive and throughresearch for improving the heat-sink property of an LED, conducted bythe present inventors, resulted in finding structures which suffer aminimum of thermal stress.

[0033] The application of the preferred embodiments of the presentinvention is best understood with reference to the accompanyingdrawings, wherein like reference numerals are used for like andcorresponding parts, respectively.

[0034] Before the present invention are disclosed or described, it is tobe understood that the terminology used therein is for the purpose ofdescribing particular embodiments only and is not intended to belimiting.

[0035] [LEDs and Fabrication Thereof]

[0036] In accordance with an aspect, the present invention pertains toLEDs which are superior in heat sink properties, and fabricationthereof.

[0037] With reference to FIG. 2, a structure of a LED is schematicallyillustrated in a cross sectional view (a) and in a plane view (b), inaccordance with an embodiment of the present invention. As seen in thesefigures, the LED of the present invention comprises a ceramic substrate11, a secondary ceramic sheet 12 located on the ceramic substrate 11, aninsulating layer 16, an upper ceramic sheet 17, an LED chip 13, andelectrodes 14.

[0038] In this embodiment, the ceramic substrate 11 has a hole for heatsink 11 a. The electrodes are patterned on the surface of the ceramicsubstrate 11 at opposite sides of the hole 11 a. Positioned below theLED chip, the hole for heat sink 11 a preferably functions toimmediately release the heat generated by the LED chip to air tominimize the thermal stress of the LED. The shape of hole for heat sinkcould be achieved by various design such as rectangle, polygons, etc.Any materials may be used as the substrate if it allows LED chips to bemounted at high density thereon. Examples of the useful ceramicsubstrate include alumina, quartz, calcium zirconate, frostbite, SiC,graphite, fused silica, mulite, cordierite, zirconia, beryllia, andaluminum nitride with preference for alumina and SiC. More preferable isalumina. Alumina ceramic is of high electrical insulation and thermalconductivity. Particularly, alumina ceramic emits less radiation inaddition to being superior in thermal resistance, chemical resistance,and mechanical strength, compared to other ceramic substrates. Further,alumina ceramic can be used for multi-layer ceramic packages (MLP),which are superior in air tightness, by forming patterns of metal wiresthereon and firing the patterned alumina ceramic.

[0039] Covering the hole for heat sink 11 a, the secondary ceramic sheet12 is positioned on the ceramic substrate to offer a site on which theLED chip is mounted. Preferably, the secondary ceramic sheet 12 is madeof alumina or SiC. The secondary sheet 12, although being formed into asquare or a diamond in FIG. 2b, is freely shaped. As will be suggestedbelow, the secondary sheet may have various shapes.

[0040] Mounted on the secondary ceramic sheet 12, the LED chip 13 iselectrically connected with the electrodes on the ceramic substrate 11via wires 15 or other patterned wires. Irrespective of its shape ortype, any LED chip can be applied to the present invention. Not only RGBLEDs, but also white LED may be used. The resulting structure is sealedwith the insulating layer 16 and the upper ceramic sheet 17. While theupper ceramic sheet 17 is formed at the periphery of the LED, theinsulating layer 16 is responsible for the central area covering the LEDchip 13, the secondary ceramic sheet 12, the wires 15 and a part of theelectrodes 16.

[0041] As for the insulating layer 16, it functions to protect the LEDchip 13 from external physical and chemical damage such as erosion,impact, etc., and is made of a transparent material so as to transmitthe light emitted from the LED chip 13. Epoxy or SiC based resins aresuitable as materials for the insulating layer 16.

[0042] With reference to FIG. 3, there are shown various structures ofLEDs in accordance with another embodiments of the present invention, inwhich reference numerals correspond to those of FIG. 2, respectively,for better understanding. Firstly, as shown in FIG. 3a, an LED chip 23is mounted on a secondary ceramic sheet 22 with further a hole 22 a forheat sink, which is positioned on a ceramic substrate 21 with a hole 21a for heat sink. For mounting, the hole for heat sink 22 a must besmaller in diameter than the LED chip 23. The hole 22 a for heat sink isalso preferably smaller than the hole for heat sink 21 a. With thisstructure, the LED chip 23 is in direct contact with air, so that theLED of FIG. 3a is anticipated to be superior in heat sink properties tothe LED of FIG. 2.

[0043]FIG. 3b illustrates a further embodiment of the present inventionconcerning an LED. In contrast to the LED suggested in FIG. 2, the LEDof FIG. 3b has such a structure that a secondary ceramic sheet 32 isformed over the entire upper surface of a ceramic substrate 31 in whichhas a hole 31 a for heat sink. In addition, electrodes 34 are located atthe secondary ceramic sheet 32, but not at the ceramic substrate 31.This LED structure is greatly advantageous for fabrication processes, aswill be explained later.

[0044] Another LED structure is shown in FIG. 3c, in accordance withstill another embodiment of the present invention. As shown in FIG. 3c,the LED is similar to that of FIG. 3b, except that a hole for heat sink42 a is further provided to a secondary ceramic sheet 42. Accordingly,the LED of FIG. 3c possesses the advantages that are found in both theLEDs of FIGS. 3a and 3 b. Concretely, this LED of FIG. 3c is furthersuperior in heat sink properties to other ones and can be alsofabricated by simpler process than a conventional process.

[0045] The above-illustrated LEDs are applicable to the case of theelectrodes suitable for use in blue or white LED chips using nitridecompound semiconductors. In the case of LED chips using othersemiconductor compounds, such as GaAs, GaP, SiC, ZnSe, etc, theirelectric conductivity allows an electric field to be applied through thetop and bottom parts of LED chips. Thus, bottom electrodes are providedbelow the LED chips. FIG. 3d shows an LED suitable for such an LED chip.The LED illustrated in FIG. 3d is quite different in electrode structurefrom those illustrated previously. As shown in FIG. 3d, an LED chip 53and a second electrode 54 a formed therebeneath are both mounted on asecondary ceramic sheet 52, and both of them are electrically connectedwith first electrodes 54 via wires 55. The ceramic substrate 51 has ahole for heat sink in this case, but the secondary ceramic sheet 52 mayhave also a hole for heat sink.

[0046] Through various designs for LED structures provided with holesfor heat sink, LEDs can be lowered in thermal stress, as explainedabove. Additionally, application of conductive materials, for example,coating or filling conductive materials to the holes for heat sink canmake the LEDs release heat at greater efficiency. Examples of suchapplications for the LED of FIG. 2 are illustrated in FIG. 4. In FIG.4a, there is a modified LED in which a metal paste 18 a is coated alongthe contact line between the secondary ceramic sheet 12 and the ceramicsheet 11 within the hole for heat sink 11 a. FIG. 4b shows an LED inwhich the hole for heat sink 11 a is filled with a metal paste 18 b. TheLED of FIG. 4c is similar to that of FIG. 4b, except that a metal plate19 is provided beneath the structure of FIG. 4b, being attached to themetal paste 18 c filling the hole for heat sink 11 a. In the LED of FIG.4d, a metal lump or slug 18 d is inserted within the hole for heat sink11 a, being attached to the secondary ceramic sheet via a metal paste.FIG. 4e shows an LED in which a metal paste 18 e is filled in the holefor heat sink, as well as being coated over the lower surface of theceramic substrate 11. The LEDs introduced in FIG. 4 can release theinternally generated heat through the metal applications attached to thehole for heat sink to the exterior more easily and thus are superior inheat sink properties, compared to the LEDs which have holes for heatsink only. Of course, such applications are applicable for the LEDs ofFIG. 3, which show diversity in designs of holes for heat sink.

[0047] Below, a detailed description will be given of the fabrication ofsuch LEDs with the LED of FIG. 2 as a central figure.

[0048] First, a ceramic sheet 11 with a hole for heat sink 11 a isprepared. The hole for heat sink 11 a may be easily formed by punching.Then, electrodes 14 are disposed on the ceramic sheet 11 by, forexample, screen printing. The patterns of electrodes may be formed invarious designs. Ag-containing paste is suitable for use as a materialfor patterning the electrodes. The ceramic substrate 11 may be of onelayer or two. In addition, the ceramic substrate, if necessary, may haveother wire patterns thereon.

[0049] After the preparation of the ceramic substrate, a secondaryceramic sheet 12, which is sufficiently large to cover the hole for heatsink 11 a, is formed on the ceramic substrate 11. In the case of the LEDof FIG. 3b, the secondary ceramic sheet 32 is as large as the ceramicsubstrate 31.

[0050] Next, there is prepared an upper ceramic sheet 17 with an openingthrough which a part of the electrodes, and a part of or all of thesecondary ceramic sheet can be exposed to the exterior when the upperceramic sheet 17 is formed on the ceramic substrate 11. Following thestacking of the upper ceramic sheet 17 on the ceramic substrate 11, thestacked ceramic substrate of the resulting structure are preferablyco-fired at about 800 to 1,050° C.

[0051] Subsequently, electrodes are formed on the patterned electrodesby plating. In this regard, Ni and Au are preferably plated on theelectrodes patterned with Ag paste on ceramic substrate, in order. Afterthe formation of electrodes, an LED chip 13 is mounted on the secondaryceramic sheet 12.

[0052] Afterwards, the electrodes are electrically connected with theLED chip via wires 15, followed by sealing the LED chip 13 with aninsulating resin 17.

[0053] [Light Emitting Device and Fabrication Thereof]

[0054] In accordance with another aspect, the present invention alsopertains to light emitting devices which employ the LEDs of various heatsink designs as elementary units and are greatly reduced in thermalstress, and fabrication thereof. The light emitting devices arefabricated not by mounting a LED chip on a PCB individually, but bymounting a multitude of LED chips on consecutive metal electrodepatterns in integrated packages.

[0055] With reference to FIG. 5, a structure of a light emitting deviceis schematically illustrated in a cross sectional view (a) and in aplane view (b), in accordance with another embodiment of the presentinvention. As seen in these figures, the light emitting device of thepresent invention generally comprises a ceramic substrate 111, aplurality of secondary ceramic sheets 112 (112′) located on the ceramicsubstrate 111, an insulating layer 116, an upper ceramic sheet 117, aplurality of LED chips 113 (113′), and electrodes 14. This lightemitting device has the structure which results from a combination ofmany LEDs of FIG. 2.

[0056] In this embodiment, the ceramic substrate 111 has a multitude ofholes for heat sink 111 a (111 a′). The electrodes are disposed on thesurface of the ceramic substrate 111 at opposite sides of each of theholes 111 a. Positioned below the LED chips 113 (113′), the holes forheat sink 111 a preferably function to immediately release the heatgenerated by the LED chips 113 (113′) to air to minimize the thermalstress of the light emitting device. The holes for heat sink 111 a (111a′) always need not be circular, but may be any form such as rectangle,polygons, etc. Examples of useful ceramic substrates, but not bylimitation, include alumina and SiC with preference for alumina.

[0057] Covering the holes for heat sink 111 a (111 a′), the secondaryceramic sheets 112 (112′) are positioned on the ceramic substrate 111 tooffer sites on which the LED chips are mounted. Preferably, thesecondary ceramic sheets 112 (112′) are made of alumina or SiC. Thesecondary sheets 112 (112′), although being formed into a square or alozenge in FIG. 5b, are freely shaped. As will be suggested below, thesecondary sheets 112 (112′) may have various shapes. The secondaryceramic sheets 112 (112′) shown in FIG. 5a are independent ones each ofwhich is located on the ceramic substrate 111, covering one hole forheat sink 111 a, but they may be formed in such a structure to cover twoor more holes for heat sink 111 a (111 a′) (see FIGS. 3b, 3 c, 10 b and11 b).

[0058] Mounted on the secondary ceramic sheets 112 (112′), the LED chips113 (113′) are electrically connected with the electrodes 114 on theceramic substrate 111 via wires 115 or other patterned wires.Irrespective of its shape or type, any LED chip can be applied to thepresent invention. Not only RGB LEDs, but also white LED may be used.The LED chips 113 (113′) are sealed with the insulating layer 116 andsurrounded by the upper ceramic sheet 117. The insulating layer 116 ispreferably made of a transparent material such as epoxy or SiC basedresin.

[0059] With reference to FIG. 6, there are shown structures of LEDdevices in accordance with another embodiment of the present invention,in which reference numerals correspond to those of FIG. 5, respectively,for better understanding.

[0060] In the light emitting device as shown in FIG. 6a, LED chips 123(123′) are mounted on secondary ceramic sheets 122 (122′) with furtherholes for heat sink 122 a (122 a′), which are positioned on a ceramicsubstrate 121 with holes 121 a (121 a′) for heat sink. This lightemitting device has the structure resulting from a combination of manyLEDs of FIG. 3a. For mounting, the holes for heat sink 122 a (122 a′)must be smaller in diameter than the LED chips 123 (123's). Also, theholes for heat sink 122 a (122 a′) are preferably smaller than the holes121 a (121 a′) for heat sink. With this structure, the LED chips 123(123′) are in direct contact with air, so that the light emitting deviceof FIG. 6a is anticipated to be superior in heat sink properties to thelight emitting device of FIG. 5.

[0061]FIG. 6b illustrates a further embodiment of the present inventionconcerning a light emitting device. In contrast to the light emittingdevice suggested in FIG. 5, the light emitting device of FIG. 6b hassuch a structure that a secondary ceramic sheet 132 is formed over theentire upper surface of a ceramic substrate 131. In addition, electrodes134 are located at the secondary ceramic sheet 132, but not at theceramic substrate 131. Having the structure resulting from a combinationof LEDs of FIG. 3b, this light emitting device structure is greatlyadvantageous for fabrication process, as will be explained later.

[0062] Another light emitting device structure is shown in FIG. 6c, inaccordance with still another embodiment of the present invention.Resulting from an assembly of LEDs of FIG. 3c, this light emittingdevice is similar to that of FIG. 3b, except that holes for heat sink142 a (142 a′) are further provided to a secondary ceramic sheet 142.

[0063] The light emitting device illustrated in FIG. 6d has thestructure resulting from a combination of LEDs of FIG. 3d. Owing to theelectrical conductivity of the substrate itself, an electric field isapplied through the top and bottom parts of LED chips in the lightemitting device. As shown in FIG. 6d, in the light emitting, LED chips153 (153′) and a second electrode 154 a (154 a′) formed therebeneath areboth mounted on secondary ceramic sheets 152 (152′) and both of them areelectrically connected with first electrodes 154 (154′) via wires 155(155′), respectively. The ceramic substrate 151 has holes for heat sink151 a (151 a′) in this case, but the secondary ceramic sheets 152 (152′)may have holes for heat sink.

[0064] Through various designs for light emitting device structuresprovided with holes for heat sink, light emitting devices can be loweredin thermal stress, as explained above. Additionally, application ofconductive materials, for example, coating or filling conductivematerials to the holes for heat sink can make the LEDs release heat athigher efficiency. Examples of such applications for the light emittingdevice of FIG. 5 are illustrated in FIG. 7. In FIG. 7a, there is amodified light emitting device in which a metal paste 118 a is coatedalong the contact line between the secondary ceramic sheet 112 and theceramic sheet 111 within the hole for heat sink 111 a. FIG. 7b shows alight emitting device in which the hole for heat sink 111 a is filledwith a metal paste 118 b. The light emitting device of FIG. 7c issimilar to that of FIG. 7b, except that a metal plate 119 is providedbeneath the structure of FIG. 7b, being attached to the metal paste 118c filling the hole for heat sink 111 a. In the light emitting device ofFIG. 7d, a metal lump or slug 118 d is inserted within the hole for heatsink 111 a, being attached to the secondary ceramic sheet 112 via ametal paste. FIG. 7e shows a light emitting device in which a metalpaste 118 e is filled in the hole for heat sink 111 a, as well as beingcoated over the lower surface of the ceramic substrate 111. The lightemitting devices introduced in FIG. 7 can release the internallygenerated heat through the metal applications attached to the hole forheat sink to the exterior more easily and thus are superior in heat sinkproperties, compared to the light emitting devices which have holes forheat sink only. Of course, such applications are applicable for thelight emitting devices of FIG. 6, which show-diversity in designs ofholes for heat sink.

[0065] Below, a detailed description will be given of the fabrication ofsuch light emitting devices with the light emitting device of FIG. 5 asa central figure, in conjunction with FIG. 8. The light emitting devicecan be fabricated by a method similar to a fabrication method for anunit LED, but its fabrication may be variously performed.

[0066] First, as shown in FIGS. 8a and 8 b, a ceramic sheet 111 with apattern of holes 111 a (111 a′) for heat sink is prepared. The holes 111a (111 a′) for heat sink on the ceramic sheet can be easily formed bypunching. Then, electrodes 114 are disposed on the ceramic sheet 111 by,for example, screen printing, as shown in FIG. 8c. The patterns ofelectrodes may be formed in various designs. Ag-containing paste issuitable for use as a material for the patterned electrodes. The ceramicsubstrate 111 may be of one ceramic sheet or more. In addition, theceramic substrate, if necessary, may have other wire patterns thereon.

[0067] After the preparation of the ceramic substrate, secondary ceramicsheets 112 (112′) are formed on the ceramic substrate 111 to cover theholes for heat sink 111 a (111 a′), respectively, as shown in FIG. 8d.The secondary ceramic sheets 112 (112′) shown in FIG. 8d are independentones each of which is located on the ceramic substrate 111, covering onehole for heat sink, but they may be formed in such a structure to covertwo or more holes for heat sink 111 a (111 a′). FIG. 9 shows an exampleof the latter case. According to a process illustrated in FIGS. 9a to 9c, an individual secondary ceramic sheet 162, which is so large as tocover a series of holes for heat sink 161 a, 161 b and 161 c, is formedon a ceramic substrate 161, followed by stacking an upper ceramic sheet167 on the ceramic substrate 161. The secondary ceramic sheets may haveholes for heat sink (see FIG. 6a).

[0068] Next, as shown in FIG. 8e, there is prepared an upper ceramicsheet 117 with openings through which a part of the patternedelectrodes, and a part of or all of the secondary ceramic sheets can beexposed to the exterior when the upper ceramic sheet 117 is formed onthe ceramic substrate 111. Following the stacking of the upper ceramicsheet 117 on the ceramic substrate 111, the resulting structure isco-fired preferably at about 800 to 1,050° C.

[0069] Subsequently, as shown in FIG. 8f, electrodes 114 are formed onthe patterned electrodes by plating. In this regard, Ni and Au arepreferably plated at the Ag paste layer on ceramic substrate, in order.After the formation of electrodes, LED chips 113 (113′) are mounted onthe secondary ceramic sheets 112 (112′)

[0070] Afterwards, the electrodes 114 are electrically connected withthe LED chips 113 (113′) via wires 115 (115′), followed by sealing theLED chips 113 (113′) with an insulating resin 116.

[0071] Turning to FIG. 10, there is illustrated a process forfabricating a light emitting device in accordance with anotherembodiment of the present invention. After completion of the process,the light emitting device will have the structure of FIG. 6c. Over thefabrication process of FIG. 8, the fabrication process illustrated inFIG. 10 has the advantage of being very simple because only onesecondary ceramic sheet is employed. This fabrication process of FIG. 10is featured in that a pattern of electrodes is not formed on a ceramicsubstrate 151 provided with holes 151 a (151 a′) for heat sink, but onthe secondary sheet 152. In detail, holes 152 a (152 a′) for heat sinkwhich are smaller than the holes 151 a (151 a′) for heat sink and theLED chips are provided for the secondary ceramic sheet 152 by punchingprocess. On the ceramic substrate is stacked the secondary ceramic sheetin such a way that the holes 152 a (152 a′) for heat sink are positionedto be concentric with the holes 151 a (151 a′) for heat sink, as shownin FIG. 10c, and then a pattern of electrodes 154 is formed on thesecondary ceramic sheet 152, as shown in FIG. 10d. Subsequent processesare the same as in FIG. 8.

[0072] In accordance with another embodiment, additional openings forheat sink, aside from holes for heat sink, may be provided to theceramic substrate. FIG. 11 shows this embodiment. As shown in FIG. 11,additional openings for heat sink 171 b are formed near the holes forheat sink 171 a (171 a′) by punching. Because the light emitting deviceof this structure can release the heat generated from LED chips througha greater area to air, a greater multitude of the light emitting devicescan be assembled into a light emitting unit assembly at a higherdensity. Additionally, the light emitting unit assembly can bestructured to have a larger area.

[0073] As for the upper ceramic sheet 177, it may be versatile inpattern. In other words, the upper ceramic sheet can be designed to havevarious patterns corresponding to given conditions, such as requirementsof end-users or use conditions. FIG. 12 shows various pattern examplesof the upper ceramic sheet. In the openings or windows of the upperceramic sheet are distributed a suitable number of LEDs according torequirements for screen areas and shapes.

[0074] [Light Emitting Unit Assembly]

[0075] At least one of the light emitting devices described above isused to construct a large area light emitting unit assembly. FIG. 13shows an example of the light emitting unit assemblies, in which theupper ceramic sheet is omitted for better understanding. The structureof the light emitting unit assembly is dependent on the distribution oflight emitting units 210, or light emitting devices. In addition, thelight emitting unit assembly can be prepared to have a desired lightemitting area and shape through the formation of the upper ceramic sheetinto the desired area and shape and thus, the intensity of the lightradiated from LED chips can be controlled. Particularly, various designsfor implementing the heat sink function can be applied to the lightemitting unit assembly of the present invention. With preferabledesigns, the heat generated from the LED chips can be easily dissipatedso that large area LED displays can be fabricated.

[0076] As described hereinbefore, the present invention offers variousdesigns suitable for efficiently dissipating the heat generated from LEDchips, providing LEDs which suffer from minimal thermal stress and canbe operated stably. In addition, the present invention provides a lightemitting device which can be applied to a large area display. The lightemitting device of the present invention can be used as a light emittingsource for full color displays, as well as substituting for conventionalluminaire, such as incandescent electric lamps, fluorescent lamps, etc.

[0077] The present invention has been described in an illustrativemanner, and it is to be understood that the terminology used is intendedto be in the nature of description rather than of limitation. Manymodifications and variations of the present invention are possible inlight of the above teachings. Therefore, it is to be understood thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

What is claimed is:
 1. A light emitting diode, comprising: a ceramicsubstrate having a hole for heat sink; a secondary ceramic sheetpositioned on said ceramic substrate, said secondary ceramic sheetcovering the hole for heat sink to mount a LED chip; electrodes disposedon said secondary ceramic sheet in a predetermined pattern around thehole; a LED chip mounted on said secondary ceramic sheet over the holeto be electrically connected with said electrodes through wires; anupper ceramic sheet formed on said secondary ceramic sheet, said upperceramic sheet surrounding said LED chip; and an insulating layer formedto seal the said LED chip within said upper ceramic sheet.
 2. The lightemitting diode as claimed in claim 1, wherein metal paste is coatedalong the contact line between the secondary ceramic sheet and theceramic substrate in the hole for heat sink.
 3. The light emitting diodeas claimed in claim 1, wherein the hole for heat sink is filled withmetal paste.
 4. The light emitting diode as claimed in claim 3, whereinthe hole for heat sink is filled with metal paste and a metal plate isattached on bottom surface of said ceramic substrate.
 5. The lightemitting diode as claimed in claim 3, wherein the hole for heat sink isfully filled with a metal paste and said ceramic substrate is entirelycoated with a metal paste on its bottom surface.
 6. The light emittingdiode as claimed in claim 1, wherein a metal lump or slug is inserted inthe hole for heat sink.
 7. The light emitting diode as claimed in claim1, wherein said ceramic substrate and/or the ceramic sheets are made ofalumina or SiC.
 8. The light emitting diode as claimed in claim 1,wherein said electrodes on the secondary ceramic sheet have a structurewhich are disposed on said secondary ceramic sheet in order of Ag/Ni/Aulayers.
 9. The light emitting diode as claimed in claim 1, wherein saidinsulating layer is made of epoxy or Si-based transparent resin.
 10. Thelight emitting diode as claimed in claim 1, wherein said secondaryceramic sheet has a hole for heat sink beneath the light emitting diodechip.
 11. A method for fabricating a light emitting diode, comprisingthe steps of: preparing a ceramic substrate having a hole for heat sink;stacking a secondary ceramic sheet on said ceramic substrate; disposingelectrode patterns in a predetermined pattern on said secondary ceramicsheet around the hole; stacking on said ceramic substrate an upperceramic sheet with an opening to form a stacked ceramic substrate insuch a way that a part of the electrodes of said secondary ceramic sheetare exposed through said opening; co-firing said stacked ceramicsubstrate; mounting a LED chip on said secondary ceramic sheet at aposition corresponding to the hole after disposing electrodes on theelectrode patterns of said secondary ceramic sheet; and sealing said LEDchip within said upper ceramic sheet with an insulating resin afterelectrically connecting said electrodes with said LED chip.
 12. Themethod as claimed in claim 11, wherein said ceramic substrate and/or theceramic sheets are made of alumina or SiC.
 13. The method as claimed inclaim 11, wherein said electrode patterns are disposed of Ag paste, andsaid electrodes are formed by plating Ni and Au on the electrodepatterns.
 14. The method as claimed in claim 11, wherein said insulatingresin is made of epoxy or an Si-based transparent resin.
 15. The methodas claimed in claim 11, wherein said co-firing is conducted at 800 to1,050° C.
 16. The method as claimed in claim 11, wherein metal paste iscoated along the contact line between said secondary ceramic sheet andsaid ceramic substrate in the hole of heat sink.
 17. The method asclaimed in claim 11, wherein the hole for heat sink is filled with metalpaste.
 18. The method as claimed in claim 17, wherein the hole for heatsink is filled with metal paste and a metal plate is attached on bottomsurface of said ceramic substrate.
 19. The method as claimed in claim17, wherein the hole for heat sink is fully filled with a metal pasteand said ceramic substrate is entirely coated with a metal paste on itsbottom surface.
 20. The method as claimed in claim 11, wherein a metallump or slug is inserted in the hole for heat sink.
 21. The method asclaimed in claim 11, wherein said secondary ceramic sheet has a hole forheat sink which is smaller than said light emitting chip and the holefor heat sink of said ceramic substrate.
 22. A light emitting diode,comprising: a ceramic substrate having a hole for heat sink, saidceramic substrate being formed electrodes in a predetermined patternthereon around the hole; a secondary ceramic sheet formed said ceramicsubstrate, said secondary ceramic sheet covering the hole for heat sinkto mount a LED chip; a LED chip mounted on said secondary ceramic sheetover the hole to be electrically connected with said electrodes throughwires; an upper ceramic sheet formed on said ceramic substrate, saidupper ceramic sheet surrounding said LED chip; and an insulating layerformed to seal said LED chip within said upper ceramic sheet.
 23. Thelight emitting diode as claimed in claim 22, wherein said secondaryceramic sheet has a hole for heat sink beneath said light emitting diodechip.
 24. A method for fabricating a light emitting diode, comprisingthe steps of: preparing a ceramic sheet having a hole for heat sink,said ceramic sheet formed electrode patterns in a predetermined patternthereon to make a ceramic substrate; stacking a secondary ceramic sheeton said ceramic substrate to cover the hole for heat sink; stacking onsaid ceramic substrate an upper ceramic sheet with an opening to form astacked ceramic substrate in such a way that a part of the electrodepatterns and a part or all of said secondary ceramic sheet are exposedthrough said opening; co-firing said stacked ceramic substrate; mountinga LED chip on said secondary ceramic sheet at a position correspondingto the hole after disposing electrodes on the electrode patterns of saidceramic substrate; and sealing said LED chip within said upper ceramicsheet with an insulating resin after electrically connecting theelectrodes with said LED chip.
 25. A light emitting diode, comprising: aceramic substrate having a hole for heat sink; a secondary ceramic sheetpositioned on said ceramic substrate, said secondary ceramic sheetcovering the hole for heat sink to mount a LED chip; a first electrodesformed in a predetermined pattern on said secondary ceramic sheet aroundthe hole; a second electrode formed just below the LED chip and on saidsecondary ceramic sheet in a predetermined pattern; a LED chip mountedon said secondary electrode of the secondary ceramic sheet over the holeto be electrically connected with said first electrodes and said secondelectrode through wires; an upper ceramic sheet formed on said secondaryceramic sheet, the upper ceramic sheet surrounding said LED chip; and aninsulating layer formed to seal said LED chip within said upper ceramicsheet.
 26. A light emitting device, comprising: a ceramic substratehaving a plurality of holes for heat sink; a secondary ceramic sheetpositioned on said ceramic substrate, said secondary ceramic sheetcovering each of the holes for heat sink to mount LED chips; electrodesdisposed in a predetermined pattern on said secondary ceramic sheetaround each of the holes; a plurality of LED chips mounted on saidsecondary ceramic sheet over each of the holes to be electricallyconnected with said electrodes through wires; an upper ceramic sheetformed on said secondary ceramic sheet, said upper ceramic sheetsurrounding said LED chips; and an insulating layer formed to seal saidLED chips within said upper ceramic sheet.
 27. The light emitting deviceas claimed in claim 26, wherein said secondary ceramic sheet has a holefor heat sink beneath said light emitting diode.
 28. A method forfabricating a light emitting device, comprising the steps of: preparinga ceramic substrate having a plurality of holes for heat sink; stackinga secondary ceramic sheet on said ceramic substrate; disposingelectrodes patterns in a predetermined pattern on said secondary ceramicsheet around each of the holes; stacking on said secondary ceramic sheetan upper ceramic sheet with an opening to form a stacked ceramicsubstrate in such a way that a part of the electrode patterns areexposed through the opening; co-firing said stacked ceramic substrate;mounting LED chips on said secondary ceramic sheet at a positioncorresponding to each of the holes after disposing electrodes on theelectrode patterns of said secondary ceramic sheet; and sealing said LEDchips within said upper ceramic sheet with insulating resin afterelectrically connecting said electrodes with said LED chips.
 29. Themethod as claimed in claim 28, wherein said secondary ceramic sheet hasholes for heat sink which are smaller than said light emitting chips andthe holes for heat sink of said ceramic substrate.
 30. A light emittingdevice, comprising: a ceramic substrate having a plurality of holes forheat sink, said ceramic substrate being formed predetermined electrodepatterns thereon around each of the holes; secondary ceramic sheetscovering each of the holes for heat sink to mount LED chips; a pluralityof LED chips mounted on said secondary ceramic sheets over each of theholes to be electrically connected with said electrode patterns throughwires; an upper ceramic sheet formed on said ceramic substrate, saidupper ceramic sheet surrounding said LED chips; and an insulating layerformed to seal said LED chips within said upper ceramic sheet.
 31. Thelight emitting device as claimed in claim 30, wherein each of saidsecondary ceramic sheets are independently positioned on said ceramicsubstrate as to cover each hole for heat sink.
 32. The light emittingdevice as claimed in claim 30, wherein each of said secondary ceramicsheets are independently positioned on said ceramic substrate to coverat least two holes or more for heat sink.
 33. A method for fabricating alight emitting device, comprising the steps of: preparing a ceramicsubstrate having a plurality of holes for heat sink, said ceramicsubstrate being formed electrode patterns in a predetermined patternthereon around each of the holes; stacking secondary ceramic sheets onsaid ceramic substrate to cover each of the holes for heat sink;stacking on said ceramic substrate an upper ceramic sheet with anopening to form a stacked ceramic substrate in such a way that a part ofsaid electrode patterns and a part or all of said secondary ceramicsheets are exposed through said opening; co-firing said stacked ceramicsubstrate; mounting LED chips on said secondary ceramic sheets at aposition corresponding to each of the holes after disposing electrodeson the electrode patterns of said ceramic substrate; and sealing saidLED chips within said upper ceramic sheet with insulating resin afterelectrically connecting said electrodes with said LED chips.
 34. Themethod as claimed in claim 33, wherein each of said secondary ceramicsheets are independently positioned on said ceramic substrate as tocover each hole for heat sink.
 35. The method as claimed in claim 33,wherein said secondary ceramic sheets are independently positioned onsaid ceramic substrate to cover at least two holes or more for heatsink.
 36. The method as claimed in claim 33, wherein said ceramicsubstrate has additional openings for heat sink, aside from the holesfor heat sink.
 37. The method as claimed in claim 33, wherein each ofsaid secondary ceramic sheets have a hole for heat sink beneath saidlight emitting diode chip.
 38. A light emitting device, comprising: aceramic substrate having a plurality of holes for heat sink; a secondaryceramic sheet positioned on said ceramic substrate, said secondaryceramic sheet covering each of the holes for heat sink to mount LEDchips; first electrodes disposed in predetermined patterns on saidsecondary ceramic sheet around each of the holes; a second electrodebeing disposed just below the LED chips and on said secondary ceramicsheet in a predetermined pattern; a plurality of LED chips mounted onthe second electrode of said secondary ceramic sheet over to beelectrically connected with said first electrodes through wires; anupper ceramic sheet formed on said secondary ceramic sheet, said upperceramic sheet surrounding said LED chips; and an insulating layer formedto seal said LED chips within said upper ceramic sheet.
 39. A large arealight emitting assembly, composed of a multitude of the light emittingdevices of claim 26.